Cladocoran A and B: Two Novel γ-HydroxybutenolideSesterterpenes from the Mediterranean Coral Cladocora cespitosa

Angelo Fontana,* Maria L. Ciavatta, and Guido Cimino

Istituto per la Chimica di Molecole di Interesse Biologico (ICMIB) del CNR, via Toiano 6,80072, Arco Felice, Napoli (Italy)1

Received August 26, 1997

The novel terpenoids, cladocoran A (1) and B (2), showing a γ-hydroxybutenolide end group, havebeen isolated from the Mediterranean Anthozoan Cladocora cespitosa. The unprecedented skeletonof cladocoran was elucidated by spectroscopic methods and chemical conversion. The absolutestereochemistry of the secondary alcohol at C-18 was assigned by the advanced Mosher’s methodapplied to an oxidized derivative of 1.

Introduction

Marine organisms have attracted considerable atten-tion as a source of novel natural products with uniquestructures and biological activities.2-4 In particular,terpenes4 show an unrivaled example of chemical diver-sity and physiologically interesting properties.5 Despitethe great interest in marine organisms, only a few studiesconcern corals of the order Madreporaria.2 Recently, wehave isolated two novel sesterterpenoids from the colonialAnthozoan Cladocora cespitosa (L.), cladocoran A (1) andB (2). In this paper we wish to describe the identificationand the remarkable chemical reactivity of 1 and 2,occurring as the main metabolites of the diethyl ethersoluble fraction from the acetone extract of the coral.

Results and Discussion

C. cespitosa (L.) is a Mediterranean organism livingalong the Italian and Spanish coasts. The Et2O extract(720 mg) of the coral was chromatographed on a silicagel column, affording several fractions, one of whichshowed two UV-absorbing products positive to a spot-reaction with cerium sulfate. Further purification of thisfraction gave the novel γ-hydroxybutenolides 1 (30 mg,Rf 0.6 petroleum/diethyl ether, 4:6) and 2 (12 mg, Rf 0.2petroleum/diethyl ether, 4:6).Treatment of cladocoran A (1) with CH2N2 in diethyl

ether gave quantitatively the optically active methyl ester3, ([R]D ) -42.0°). Compound 3 exhibited an EIMSmolecular ion atm/z 472 corresponding to the molecularformula C29H44O5 (HREIMS m/z 472.3165). The carbonresonances at δ 169.4 and 166.4 and the IR absorptionsat 1744 and 1722 cm-1 indicated two ester functions, oneof which was assigned to a â,â′-disubstituted R,â-

unsaturated moiety on the basis of the proton signal atδ 5.94 and the UV band centered at 224 nm. The 1HNMR spectrum of 3 was poorly defined in CDCl3 due tosignal overlapping (Table 1). However, it was satisfac-torily resolved whenmeasured in C5D5N. The resonancesat δ 6.22 (H-21), 5.56 (H-10), 4.87 (H-3a), and 4.83 (H-3b) in the 1H NMR (C6D5N) spectrum, as well as the six13C NMR signals in the olefinic region, implied twotrisubstituted double bonds and one exomethylene groupthat accounted for three of the eight unsaturationsrequired. The broad signal at δ 4.85 (H-20) was coupledto the methylene protons at δ 3.26 (H-26a) and 3.11 (H-26b) and showed long-range connectivities with thecarbons at δ 160.8 (C-19), 117.7 (C-21), and 47.5 (C-26)in the HMBC experiments (Figure 1). Moreover, consis-tent with partial structure c, diagnostic correlations werealso observed between the olefinic carbon at δ 160.8 (C-19) and the protons at δ 6.22 (H-21) and 4.96 (H-18). The

* To whom correspondence should be addressed. Phone: ++39 818534156. Fax: ++39 81 8041770. E-mail address: Font@trinc.icmib.na.cnr.it.

(1) Associated with the National Institute for the Chemistry ofBiological Systems (CNR).

(2) Faulkner, D. J.Nat. Prod. Rep. 1997, 14, 259-302 and referencestherein.

(3) Cragg, G. M.; Newman, D. J.; Snader, K. M. J. Nat. Prod. 1997,60, 52-60.

(4) Kobayashi, J.; Ishibashi, M. Chem. Rev. 1993, 93, 1753-1769.(5) Mann, J.; Davidson, R. S.; Hobbs, J. B.; Banthorpe, D. V.;

Harborne, J. B. In Natural products: their chemistry and biologicalsignificance; Longaman Scientific & Technical Ed.: Hong Kong, 1994;pp 289-358.

2845J. Org. Chem. 1998, 63, 2845-2849

S0022-3263(97)01586-7 CCC: $15.00 © 1998 American Chemical SocietyPublished on Web 04/07/1998

acetoxy function, suggested by the sharp singlet at δ 2.03in the 1H NMR spectrum, was assigned at C-18 (δ 69.3)on the basis of the downfield shift (δ 4.96) of the carbinolproton, whereas the COSY cross-peaks allowed linkageC-18 to C-17 (δ 1.38 and 2.20). The structure of the C1-C6 side chain (partial structure a) was defined by thepresence of the C3-C5 system in the TOCSY spectrumand by the 2,3JC,H correlations of the quaternary carbonat δ 146.4 (C-2) with the protons at δ 2.10 (H2-4), 1.73(H3-1), and 1.50 (H2-5) (Figure 1). A more complexanalysis was required for the elucidation of the centralbicyclic system (partial structure b) (Chart 1). Irradia-tion of the broad triplet at δ 5.56 (H-10) indicated anallylic coupling with the signal at δ 2.19 (H-12) and a

vicinal coupling with that at δ 1.95 (H2-9). This latterresonance revealed a further interaction with the meth-ylene group centered at δ 1.33 (H2-8). On the other hand,TOCSY data connected clearly the carbon chain from C12

to C15 and confirmed the results of the COSY experimentwhich showed cross-peaks between the adjacent protonsH-12/H2-13, H2-13/H2-14, and H2-14/H-15. Double reso-nance and COSY experiments also indicated couplingbetween the methyl doublet at δ 0.84 (H3-24) and themethine at δ 1.22 (H-15). The HMBC spectrum wasdiagnostic in closing the bicyclic system and in linkingthe partial structures a, b, and c. The quaternary carbonat δ 34.4 (C-7) showed long-range correlations with theprotons at δ 2.19 (H-12), 1.63 (H-6a), 1.35 (H-6b and H2-8), 1.50 (H2-5), and 1.95 (H2-9), as well as with the methylgroup at δ 0.81 (H3-23) (Figure 1). In the same way, themethine proton at δ 1.22 (H-15) was correlated to thequaternary carbons at δ 42.9 (C-16) and 144.2 (C-11), andthe signal at δ 2.19 (H-12) to the C-11 (δ 144.2). Finally,cross-peaks from C-16 (δ 42.9) to the methyl at δ 1.28(H3-25) and the methylene protons at C-17 (δ 1.38 and2.20) connected b and c, and completed the skeletalassignment (Table 1).The relative stereochemistry of the bicyclic system in

3 was established by analysis of vicinal coupling con-stants (3Jvic) and of NOEs (Figure 2) in three differentNOESY experiments. The proton at C-12 showed anaxial-axial (JH-12/H-13a ) 12.5 Hz) and an axial-equato-rial (JH-12/H-13b ) 5.5 Hz) coupling constant with themethylene group at C-13, whereas H-15 occurred as abroad signal exhibiting two small couplings with H2-14

Table 1. 1H and 13C NMR Dataa of Cladocoran A Methyl Ester (3) in C5D5N and CDCl3

C5D5N CDCl3

position Hb Cc HMBC Hb Cc

1 1.73 (br s, 3H) 22.6 q H-3a, H-3b, H-4 1.72 (br s, 3H) 22.6 q2 146.4 s H-1, H-3a, H-3b, H-4, H-5 146.5 s3 4.83 (s, 1H) 110.2 t H-1, H-4 4.69 (s, 2H) 109.3 t

4.87 (s, 1H)4 2.10 (m, 2H) 38.9 t H-1, H-3a, H-3b, H-5, H-6b 2.02 (m, 2H) 38.5 t5 1.50 (m, 2H) 21.9 t 1.43 (m, 2H) 21.5 t6 1.35 (m, 1H) 39.8 t H-4, H-5, H-23 1.23 (m, 1H) 39.4 t

1.63 (m, 1H) 1.43 (m, 1H)7 34.4 s H-5, H-6a, H-6b, H-8, H-12, H-23 34.1 s8 1.33 (m, 2H) 30.2 t H-6a, H-9, H-10 1.30 (m, 2H) 29.3 t9 1.95 (m, 2H) 23.4 t H-8, H-10 1.98 (m, 2H) 23.1 t10 5.56 (br t, 1H) 119.6 d H-8 5.43 (br t, 1H, 3.4) 119.2 d11 144.2 s H-9, H-12, H-13, H-15, H-25 143.8 s12 2.19 (m, 1H) 43.3 d H-6a, H-6b, H-10, H-13a, H-23 1.99 (m, 1H) 43.0 d13 1.05 (ddd, 1H, 5.5, 5.5, 12.5) 29.1 t H-12, H-14 1.05 (ddd, 1H, 3.8, 4.2, 12.9) 28.7 t

1.80 (bdd, 1H, 12.5, 3.1) 1.80 (m, 1H)14 1.47 (m, 2H) 31.5 t H-13, H-15, H-24 1.35 (m, 1H) 31.2 t

1.56 (m, 1H)15 1.22 (m, 1H) 44.9 d H-12, H-13a, H-13b, H-14, H-24 1.23 (m, 1H) 44.6 d16 42.9 s H-10, H-14, H-15, H-17a, H-17b, H-24, H-25 42.2 s17 1.38 (dd, 1H, 3.0, 14.6) 37.1 t H-18, H-25 1.30 (m, 1H) 36.7 t

2.20 (m, 1H) 2.05 (m, 1H)18 4.96 (m, 1H) 69.3 d H-17a, H-20, H-21 4.71 (d, 1H, 7.0) 69.1 d19 160.8 s H-17a, H-17b, H-18, H-20, H-21, H-26a, H-26b 160.6 s20 4.85 (br s, 1H) 50.2 d H-18, H-21, H-26a, H-26b 4.57 (t, 1H, 3.5) 49.7 d21 6.22 (s, 1H) 117.7 d H-18 5.94 (s, 1H) 116.9 d22 166.5 s H-21, MeCOO 166.4 s23 0.81 (s, 3H) 22.9 q H-6b 0.79 (s, 3H) 22.7 q24 0.84 (d, 3H, 7.0) 16.7 q H-15 0.83 (d, 3H, 6.7) 16.4 q25 1.28 (s, 3H) 25.9 q H-15, H-17a, H-17b 1.16 (s, 3H) 25.5 q26 3.11 (t, 1H, 4.6) 47.5 t 3.03 (d, 2H, 3.5) 47.2 t

3.26 (dd, 1H, 4.6, 2.2)MeCOO 3.65 (s, 3H) 51.4 q 3.71 (s, 3H) 51.4 qMeCO 2.03 (s, 3H) 20.9 q 2.01 (s, 3H) 21.2 q

169.9 s 169.4 sa Assignments aided by 1H COSY, 1H-1H decoupling, and HMQC experiments. b 500 MHz. c 125 MHz.

Figure 1. HMBC for compound 3.

Chart 1

2846 J. Org. Chem., Vol. 63, No. 9, 1998 Fontana et al.

(Figure 2). A strong enhancement (16%) was experiencedby CH3-25 when H-10 had been irradiated, and a cisrelationship between CH3-25 and CH3-24 was attributedon the basis of a cross-peak in the NOESY spectrum. Aweak but very diagnostic NOE between H-12 and themethylene hydrogens at C-6 defined the relative stere-ochemistry of the alkyl chain at C-7. We assigned allthe methyl groups as â on biogenetic grounds, but it isimpossible to exclude the possibility that the orientationof CH3-23 is opposite to that of CH3-24 and CH3-25.Once the novel skeleton of 3 was elucidated, the

structure of the natural precursor 1 was easy to assign(Table 2). Cladocoran A (1) is an amorphous colorlessoil, [R]D ) -25.8°. Its molecular formula was determinedto be C27H40O5 by HREIMS (444.2875) and differs fromthat of 3 by the loss of 28 Da. The missing units werethose of the ester methyl group and of the methylenemoiety in the oxirane ring. The 13C NMR in CDCl3 (Table2) contained 27 signals including seven quaternarycarbons and five methyl, nine methylene, six methinegroups, of which one was a vinylic carbon (δ 118.4, C-21),

one a hemiacetal carbon (δ 97.5, C-20), and another ahydroxyl-bearing carbon (δ 67.5, C-18) (Table 2). Theassignment of dehydrodecaline system resonances as wellas those of the side chain in compound 3 were enterelysupported by 2D-NMR experiments and confirmed bycomparison with data in 1. In the same way, the relativestereochemistry of the ring substituents in cladocoran A(1) was determined to be the same as that found in 3 byNOESY and monodimensional NOE experiments. Theremaining NMR data of 1, which greatly differed fromthose of 3, suggested the presence of a γ-hydroxybuteno-lide inserted in a molecular arrangement related to thatof secomanoalide (7)8 or of luffariellin B (8).9 In fact, theHMBC spectrum revealed long-range correlations be-tween H-21 and C-22 (δ 168.2), C-19 (δ 170.1) and C-20(δ 97.5), as well as cross-peaks of C-19 with H-20 andH-18. This last proton also showed COSY correlationsboth with the methylene protons at δ 1.72 and 1.16 (Hs-17) and with olefin hydrogen at δ 5.95 (H-21). Finally,as occurred in 3, the long-range correlations of H2-17 withC-16 (δ 42.8) and CH3-25 (δ 24.7) allowed the side chainto be linked to C-16 and completed the unambiguousstructural characterization of the sesterterpene 1.It is obvious that the γ-hydroxy γ-lactone end moiety

of cladocoran A (1) arises from the intramolecular addi-tion of the carboxylic acid to an aldehyde group at C-20

(6) Bax, A.; Subraminian, S. J. J. Magn. Reson. 1986, 67, 565-569.(7) Bax, A.; Sommers, M. F. J. Am. Chem. Soc. 1986, 108, 2093-

2094.(8) de Silva, E. D.; Scheuer, P. J. Tetrahedron Lett. 1981, 22, 3147-

3150.(9) Kernan, M. R.; Faulkner, D. J.; Jacobs, R. S. J. Org. Chem. 1987,

52, 3081-3083.

Table 2. 1H and 13C NMR Dataa of Compounds 1 and 2

1 2

position Hb Cc HMBC Hb Cc

1 1.72 (br s, 3H) 22.4 q 1.70 (br s, 3H) 22.3 q2 146.9 s H-1 146.8 s3 4.70 (s, 2H) 109.5 t H-1 4.65 (s, 1H) 109.8 t

4.71 (s, 1H)4 2.02 (m, 2H) 38.3 t H-1 2.01 (m, 2H) 38.3 t5 1.40 (m, 2H) 21.5 t 1.45 (m, 2H) 21.3 t6 1.05 (m, 1H) 39.2 t H-23 1.05 (m, 1H) 38.8 t

1.40 (m, 1H) 1.40 (m, 1H)7 34.1 s H-9, H-23 33.9 s8 1.30 (m, 2H) 29.3 t H-23 1.25 (m, 1H) 29.0 t

1.35 (m, 1H)9 1.95 (m, 2H) 22.9 t H-8 2.02 (m, 2H) 22.7 t10 5.55 (br t, 1H) 120.5 d H-9, H-17, H-25 5.64 (br t, 1H) 119.7 d11 143.3 s H-25 143.4 s12 1.58 (m, 1H) 44.1 d H-23 1.58 (m, 1H) 43.5 d13 1.08 (m, 1H) 29.7 t 1.10 (m, 1H) 29.3 t

1.80 (m, 1H) H-24 1.82 (m, 1H)14 1.35 (m, 1H) 31.3 t 1.35 (m, 1H) 31.2 t

1.61 (m, 1H) H-24, H-25 1.60 (m, 1H)15 1.30 (m, 1H) 44.7 d H-24, H-25 1.30 (m, 2H) 44.8 d16 42.8 s H-24, H-25 42.6 s17 1.16 (m, 1H) 34.7 t H-25 1.47 (m, 1H) 37.1 t

1.72 (m, 1H) 2.05 (m, 1H)18 5.19 (d, 1H, 10.6) 67.5 d H-17, H-21 4.54 (d, 1H, 10.5) 66.4 d19 168.4 s H-18 170.0 s20 6.05 (br s, 1H) 97.5 d H-18, H-21 6.11 (br s, 1H) 97.1 d21 5.95 (s, 1H) 118.4 d H-18 6.01 (s, 1H) 117.2 d22 169.0 s H-20, H-21 171.0 s23 0.82 (s, 3H) 23.3 q 0.83 (s, 3H) 23.7 q24 0.88 (d, 3H, 7.0) 16.4 q 0.86 (d, 7.0, 3H) 16.4 q25 1.16 (s, 3H) 24.7 q 1.26 (s, 3H) 25.7 qMeCO 2.08 (s, 3H) 20.9 q

171.0 s H-18, Mea Assignments aided by 1H COSY, 1H-1H decoupling, and HMQC experiments. b 500 MHz. c 125 MHz.

Figure 2. Selected NOE for compound 3.

Synthesis of Cladocoran A and B J. Org. Chem., Vol. 63, No. 9, 1998 2847

(Chart 2). As suggested by the small signal at δ 9.41 inthe 1H NMR spectrum (see Supporting Information), thecyclic structure seems to be in equilibrium with the openform 1a. Our efforts to isolate the native aldehyde werefruitless, but the presence of this compound explainssatisfactorily the formation of compound 3 (Chart 2).Additionally, the reduction10 of 1 with NaBH4 in MeOHgave the γ-butenolide 4 as the major product (seeExperimental Section). The lactone 4 had the molecularformula C27H40O4 (EIMS m/z 428) and showed an ABsystem (δ 4.71 and 4.82, J ) 16.5) for H2-20, as well asthe expected upfield-shift of C-20 (δ 70.8) in the 13C NMRspectrum (see Experimental Section).Cladocoran A (1) occurs in the organic extract as a

mixture of R and â epimers at C-20. In C5D5N (seeSupporting Information), the signals of H-20 for the Rand â diasteroisomers appeared resolved and of similarintensities. This was confirmed by GC-MS and HPLCanalysis of the mixture of epimeric alcohols 5a and 5bobtained by acetylation of 1 (see Experimental Section).The absolute configuration of the cladocoran skeleton

at C-18 was deduced by Mosher’s method.10,11 As thehydrolysis of the acetyl function of 1 in alkaline or acidicmedium gave an unexpectedly complex mixture of prod-ucts, the absolute stereochemistry was established byMTPA esterification of the derivative 6. This latestcompound resulted from treatment of the product 3 withNa2CO3 in dry MeOH. It is likely that 6 is formed by atransesterification involving the secondary alcohol de-

rived from the opening of the oxirane ring (Chart 3).Reaction of 6 with (R)- and (S)-MTPA chlorides in drypyridine yielded the corresponding (S)-MTPA (6a) and(R)-MTPA (6b) esters. The R absolute configuration ofC-18 was assigned on the basis of the ∆δ values11,12calculated by the 1H NMR and COSY spectra of 6a and6b in CDCl3 (Figure 3).Cladocoran B (2) was isolated as a colorless oil. Its

molecular weight was determined at m/z 402, corre-sponding to the molecular formula C25H38O4 (HREIMS402.2774 m/z). All spectral data for 2 were very similarto those of 1, except for the absence of the acetyl groupat C-18 (Table 2). The upfield shift of H-18 (δ 4.54 ppm)in 2 supported the proposed structure of cladocoran B(2). This assignment was further confirmed by theacetylation of 2 with acetic anhydride in pyridine to give1.

Conclusion

The extracts of the colonial organism C. cespitosacontain the unique sesterterpenes 1 and 2 featuring theγ-hydroxybutenolide group previously associated withphospholipase A2 inhibition.16-19 Although characterizedby a novel carbon skeleton, cladocorans share more than

(10) Midland, S. L.; Wing, R. M.; Sims, J. J. J. Org. Chem. 1983,48, 1906-1909.

(11) Dale, J. A.; Mosher, H. S. J. Am. Chem. Soc. 1973, 95, 512-515.

(12) Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am.Chem. Soc. 1991, 113, 4092-4097.

(13) Hagiwara, H.; Uda, H. J. Chem. Soc. Chem. Commun. 1988,815-817.

(14) de Silva, E. D.; Scheuer, P. J. Tetrahedron Lett. 1980, 21, 1611-1614.

(15) Potts, B. C. M.; Capon, R. J.; Faulkner, D. J. J. Org. Chem.1992, 57, 2965-2967.

(16) Conte, M. R.; Fattorusso, E.; Lanzotti, V.; Magno, S.; Mayol, L.Tetrahedron 1994, 50, 849-856.

Figure 3. ∆δ observed for MTPA [δS ester - δR ester] derivativesof 6. Spectra were recorded in CDCl3 at 25 °C.

Chart 2

Chart 3

2848 J. Org. Chem., Vol. 63, No. 9, 1998 Fontana et al.

one analogy with the aldose reductase inhibitor dyside-apaluanic acid (9)13 and the protein phosphatase inhibitordysidiolide (10),20 both recently isolated from the organicextracts of sponges Dysidea.Regarding the origin of cladocorans, some doubts still

remain about the actual organism able to synthesizethem. The cladocoran skeleton, as previously stated, isclosely related to compounds typically found in sponges.Moreover, the calcareous exoskeleton of the Cladocoracollected at Aguilas offers hospitality to a population ofgreen algae and, very likely, other organisms, such asbacteria or encrusting sponges. Extracts of the coral fromother Mediterranean sites (south of Spain and southeastof Italy) do not contain 1 and 2, thus supporting the thesisof their origin from symbionts or sponges.

Experimental Section

Collection. The colonial Anthozoan C. cespitosa (L.) wascollected off Aguilas (south of Spain) at a depth of 20 m, inSeptember 1995. The colonial organisms (250 g dry weight),frozen immediately, were transferred to ICMIB and stored at-20 °C until the analysis day. A voucher specimen isdeposited in our institute.Extraction and Isolation. The frozen animals were

chopped three times, and extracted with acetone (3× 500 mL).The acetone extracts were filtered and reduced in vacuo to anaqueous layer (ca. 400 mL) and partitioned with diethyl ether(4 × 300 mL). At once, the ethereal fractions were evaporatedin vacuo to yield ca. 720 mg of extract. A silica gel column(200 g) was packed with light petroleum ether and eluted withincreasing amounts of diethyl ether. Fractions obtained fromlight petroleum ether/diethyl ether 6:4 (100 mg) and lightpetroleum ether/diethyl ether 3:7 (60 mg) were submitted toanother purification on silica gel column (15 g), achieving purecladocoran 1 (30 mg) and 2 (12 mg).Cladocoran A (1): pale yellow oil; [R]20D ) -25.8° (c 0.4,

CHCl3); UV λmax (EtOH) 224 (ε 8400); IR νmax (liquid film) 3367,2935, 2865, 1762, 1753, 1645, 1452, 1375, 1228, 1135, 888, 764;EIMS, m/z (%) 444 (3, M+), 429 (3, M+ - 15), 384 (25, M+ -60), 301 (20, M+ - 60 - C6H11), 257 (90), 213 (90); HREIMS,m/z 444.2887 (C27H44O5 requires 444.2875); 1H and 13C NMRdata, see Table 2.Cladocoran B (2): pale yellow oil; [R]20D ) -59.9° (c 0.6,

CHCl3); UV λmax (EtOH) 224 (ε 8700); IR νmax (liquid film) 3364,2934, 2873, 1748, 1649, 1456, 1139, 954, 890, 759; EIMS, m/z(%) 402 (3, M+), 384 (3, M+ - H2O), 366 (3, M+ - 2 H2O), 301(50, M+ - H2O - C6H11), 259 (70), 201 (65), 176 (100);HREIMS,m/z 402.2774 (C25H38O4 requires 402.2769); 1H and13C NMR data see Table 2.

Cladocoran A Methyl Ester (3). To 10 mg of cladocoranA (1) was added 5 mL of diazomethane in diethyl ether. After0.5 h the diethyl ether was evaporated in vacuo and 3 wasquantitatively obtained: pale yellow oil; [R]20D - 42.0° (c 0.2,CHCl3); UV λmax (EtOH) 224 nm (ε 12 000); EIMS,m/z (%) 472(5, M+), 457 (5), 387 (15), 329 (20), 311 (30), 258 (80), 213 (100);HREIMS, m/z 472.3165 (C29H44O5 requires 472.3188); 1H and13C NMR data see Table 1.Reduction of Cladocoran A (1) in 4. A 5 mg portion of

compound 1 was treated with MeOH and NaBH4 as previouslydescribed:10 oil; [R]20D ) -38.3 (c 0.2, CHCl3); IR νmax (liquidfilm) 2926, 2857, 1792, 1753, 1645, 1451, 1375, 1227, 1151,1027, 895, 772; EIMS,m/z (%) 1H NMR data (CHCl3, 500 MHz)δ 1.70 (s, H3-1), 4.63 (s, 1H, H-3a), 4.70 (s, 1H, H-3b), 2.02 (m,2H, H-4), 1.40 (m, 2H, H-5), 1.05 (m, 1H, H-6a), 1.39 (m, 1H,H-6b), 1.25 (m, 1H, H-8a), 1.35 (m, 1H, H-8b), 1.98 (m, 2H,H2-9), 5.50 (br t, 1H, H-10), 1.58 (m, 1H, H-12), 1.11 (m, 1H,H-13a), 1.82 (m, 1H, H-13b), 1.23 (m, 1H, H-14a), 1.61 (m, 1H,H-14b), 1.31 (m, 1H, H-15), 1.70 (m, 1H, H-17a), 1.95 (m, 1H,H-17b), 5.39 (d, 10.6, 1H, H-18), 4.71-4.82 (ABq, J ) 16.5 Hz,H2-20), 5.89 (br s, 1H, H-21) 0.82 (s, 3H, H-23), 0.87 (d, 7.1,3H, H-24), 1.12 (s, 3H, H-25), 2.08 (s, 3H, CH3CO); 13C data(125 MHz, CDCl3) 22.3 (C-1), 145.9 (C-2), 109.9 (C-3), 38.3 (C-4), 21.4 (C-5), 39.1 (C-6), 33.9 (C-7), 29.1 (C-8), 23.7 (C-9), 120.5(C-10), 143.4 (C-11), 43.8 (C-12), 29.5 (C-13), 31.6 (C-14), 44.7(C-15), 42.5 (C-16), 35.4 (C-17), 5.39 (C-18), 169.7 (C-19), 70.8(C-20), 115.5 (C-21), 169.7 (C-22), 22.9 (C-23), 16.4 (C-24), 24.4(C-25), 20.9 (CH3CO).Treatment of 1 with Ac2O. To 3 mg of cladocoran A (1)

was added Ac2O in dry pyridine and stirred at rt overnightaffording a mixture, 5a and 5b. The mixture of 5a and 5bwas injected on a Spherisorb SW5 HPLC column (n-hexane/ethyl acetate, 95:5, flow 0.8 mL/min, detector UV 254 nm) andtwo main peaks in 1:1 ratio were observed.MTPA Esters. In order to establish the absolute stereo-

chemistry of C-18, cladocoran A methyl ester (3) was firstsubmitted to methanolysis (MeOH anhydrous and Na2CO3

overnight) and then treated with (R)-(-)-MTPA chloride and(S)-(+)-MTPA chloride to yield S- and R-esters of 6, respec-tively.S-ester 1H NMR data: 5.97 (H-21), 5.65 (H-18), 5.49 (H-

10), 5.19 (H-20a), 4.99 (H-20b), 0.82 (H-24), 1.05 (H-25).R-ester 1H NMR data: 6.07 (H-21), 5.67 (H-18), 5.36 (H-

10), 5.24 (H-20a), 5.07 (H-20b), 0.87 (H-24), 0.88 (H-25).

Acknowledgment. We thank Dr. Guido Villani andMr. Gennaro Scognamiglio for collection and technicalassistance. Mass and NMR spectra were obtained from“Servizio di Massa del CNR e dell′Universita di Napoli”and “Servizio NMR dell′Area CNR di Napoli”, respec-tively, the staff of both of which are acknowledged. Thiswork was partly supported by a CNR/CSIC Italian-Spanish bilateral project.

Supporting Information Available: 1H and 13 C NMRspectra of 1, 1a, 2, and 5a,b (13 pages). This material iscontained in libraries on microfiche, immediately follows thisarticle in the microfilm version of the journal, and can beordered from the ACS; see any current masthead page forordering information.

JO971586J

(17) Lombardo, D.; Dennis, E. A; J. Biol. Chem. 1985, 260, 7234-7240.

(18) Sullivan, B.; Faulkner, D. J. Tetrahedron Lett. 1982, 23, 907-910.

(19) De Rosa, S.; De Stefano, S.; Zavodnik, N. J. Org. Chem. 1988,53, 5020-5023.

(20) Gunasekera, S. P.; McCarthy, P. J.; Kelly-Borges, M.; Lobkov-sky, E.; Clardy, J. J. Am. Chem. Soc. 1996, 118, 8759-8760.

Synthesis of Cladocoran A and B J. Org. Chem., Vol. 63, No. 9, 1998 2849